The present invention relates generally to cooling gas turbines, for example, for electrical power generation and, more particularly, to the provision of a metering plate in an impingement insert for metering flow into that impingement insert.
The traditional approach for cooling turbine blades and nozzles is to use high pressure cooling air extracted from a source, such as from the intermediate and last stages of the turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. External piping is generally used to supply air to the nozzles, with the air typically exiting into the hot gas stream of the turbine to provide air film cooling of the nozzle surface.
In advanced gas turbine designs, it was recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It was therefore necessary to develop a cooling scheme that more assuredly protects the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Certain areas of the components of the hot gas path, however, cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges. Therefore, air cooling may be provided in the trailing edges of nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference.
In turbine nozzles there are typically impingement inserts disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. Metering plates may be used in an impingement cooled multiple cavity nozzle to balance the total cooling system overall flow distribution. The use of a metering plate in an impingement insert creates a flow disruption that causes reduced total pressure in the area just below the metering plate and therefore meters the flow into that particular impingement insert. A typical metering plate has one or more orifice holes to control the flow into the insert. Conventionally, the metering plate is placed on top of the insert inlet prior to or during assembly into the nozzle. Because of the reduced total pressure in the area below the metering plate, the metering plate reduces the pre-impingement pressure in the insert thereby reducing ballooning stresses in the insert.
The present invention provides a cooling system for cooling the hot gas components of a nozzle stage of a gas turbine, in which closed circuit steam or air cooling and/or open circuit air cooling systems may be employed. In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. This closed circuit cooling system is substantially structurally similar to the steam cooling system described and illustrated in the prior referenced U.S. Pat. No. 5,634,766, with certain exceptions as noted below. Thus, cooling media is provided to a plenum in the outer wall of the segment for distribution therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. Return intermediate cooling cavities extend radially and lie between the leading edge and aft cavities. A separate trailing edge cavity may also be provided.
The cooling media that flows through the leading edge and aft cavities flows into a plenum in the inner wall and through impingement openings in an impingement plate for impingement cooling of the inner wall of the segment. The spent impingement cooling media then flows through the intermediate return cavities for further cooling of the vane.
Impingement cooling is also provided in the leading and aft cavities of the nozzle vane, as well as in the intermediate, return cavities of the vane. More specifically, impingement inserts are disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. The inserts in the leading and aft cavities comprise sleeves having a collar at their inlet ends for connection with integrally cast flanges in the outer wall of the cavities and extend through the cavities spaced from the walls thereof. These inserts have impingement holes in opposition to the walls of the cavity whereby cooling media, e.g. steam, flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Return or exit channels may be provided along the inserts for channeling the spent impingement cooling media. Similarly, inserts in the return intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane. These inserts also may have return or exit channels for collecting the spent impingement cooling media and conducting it to the cooling media outlet.
Typically, nozzles do not have metering plates as a part of the impingement insert design. Of the known designs using inserted metering plates, the metering plate is placed on and connected to the top of the insert. As used herein, xe2x80x98top of the insertxe2x80x99 refers to the entrance end or inlet end of the insert with respect to the direction of coolant flow therethrough. Thus, intermediate inserts through which coolant flow flows radially outwardly would have a metering plate, if provided, disposed at a radially inner end thereof.
While metering plates are considered generally effective to balance cooling flow to different cavities of the nozzle as required and to reduce ballooning stresses on the insert, that is not to say that improvement thereof cannot be made. Indeed, in an embodiment of the invention, by relocating the metering plate from its conventional top end placement, significant improvements to the flow distribution and insert assembly can be achieved.
Thus, in an embodiment of the invention, an impingement insert metering plate is provided that improves the mechanical robustness of the insert assembly, improves the assembly of the insert to the nozzle and improves the manufacturing assembly of the insert.
Accordingly, the invention is embodied in an impingement insert sleeve for being disposed in a coolant cavity defined through a stator vane, the insert having a generally open inlet end and first and second diametrically opposed, perforated side walls. A metering plate having at least one opening defined therethrough for coolant flow is mounted to the side walls to generally transverse a longitudinal axis of the insert, downstream from said inlet end.
The invention is further embodied in a turbine vane segment, comprising inner and outer walls spaced from one another; a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium; an insert sleeve within one the cavity and spaced from interior wall surfaces thereof, the insert sleeve having an inlet end through which cooling medium flows into the insert sleeve, the insert sleeve having a plurality of openings therethrough for flowing the cooling medium through the openings into the space between the sleeve and the interior wall surfaces for impingement against the interior wall surface of the vane; and a metering plate having at least one opening for cooling medium flow defined therethrough, the metering plate being mounted to the insert sleeve so as to substantially traverse a flow path defined therethrough, the metering plate being spaced from the inlet end of the insert sleeve.